Bicycles in general are very well known, and provide an exceptionally valuable mode of transportation. However, they require balance to maneuver and a certain amount of agility to keep upright and going on a straight course. For the very young--and very old--cyclists, and for people who have insufficient strength or balance to maneuver a bicycle, the tricycle provides a stabler and safer mode of transportation, while still retaining some of the advantages of a bicycle.
Most tricycles are designed for ordinary people who have reasonable use of all of their limbs, and reasonable balance for getting on and off of the tricycle, and for sitting on a conventional saddle that can be adjusted, to a limited degree, for driving the tricycle in a well known manner.